Optical amplifiers are used widely in optical systems, particularly optical transmission systems. In wavelength division multiplexed (WDM) systems they operate over a wide wavelength range, typically 1520 nm to 1570 nm, and in some systems to 1620 nm. Most optical amplifiers are known to produce non-uniform gain outputs over this broad range. Erbium-doped fiber amplifiers (EDFAs) are widely used in optical communications systems, but, while effective in terms of performance, size, cost, and reliability, erbium-doped fiber itself produces gain curves that are not only non-linear, but have wide swings with multiple inflections. Typically the gain curve produced by erbium-doped fiber has a maxima around 1530 and at least one other around 1560.
Raman amplification, widely used in undersea cables, also produce non-uniform gain curves.
The non-uniformities are usually addressed by providing gain flattening filters (GFFs, sometimes referred to as gain equalizing filters) within or at the output of the optical amplifier. Since the gain curves are highly non-uniform, some portions of the gain curve require flattening while others may not. Accordingly GFFs are frequently tailor made to produce an attenuation curve that is the inverse of the amplifier gain curve.
Optical filters come in a variety, of forms. The most common are Thin Film Filters (TFF), Array Waveguides (AWG), Long Period Gratings (LPGs) and Fiber Bragg Gratings (FBGs). For a variety of reasons, TFFs are the most versatile. Combinations of TFFs are available for all common gain flattening filter applications. Moreover, TFFs provide:                Thermal stability        Superior optical properties including                    Low insertion loss            Wide and flat passband            Excellent isolation            Small polarization dependent loss                        Modularity and scalability        Low cost through cost effective manufacturing using batch processes        
The drawback to TFFs for gain flattening is that the filter is typically custom made to filter a specific optical spectrum. Thus, while TFFs can be easily made for essentially any spectrum, they are not adjustable in the event the spectrum changes.
A competing option for flattening amplifier gain curves is Dynamic Gain Equalization (DGE). Devices using this approach have a diffraction grating in combination with an array of MEMS or LC tuning elements to create dynamically settable GFF responses. Since they are adjustable they can account for changes in the gain curve of the amplifier(s). However, DGE devices are complex, expensive and have high loss. Consequently, DGEs are typically used after multiple amplifiers stages to equalize the composite gain.
A simpler, cost effective solution, based on TFFs, but with dynamic control would be a significant contribution to the technology. Moreover, if the cost is low, integrating a GFF device with each single amplifier stage would be cost effective.